What is the primary cause of shoulder dystocia during childbirth?

The primary cause of shoulder dystocia during childbirth is when the infant's shoulders become lodged behind the pubic symphysis after the head has been delivered. This situation occurs due to the baby's shoulder width being broader than the pelvic opening it needs to navigate. As the shoulders pass through the birth canal, they can get stuck if their circumference exceeds the available space, leading to a delay in full delivery and potential complications for both the mother and baby. This scenario underscores the mechanics of childbirth and highlights the structural challenges that can arise during delivery.

The other options, while they may relate to complications in childbirth, do not specifically address the mechanics of shoulder dystocia. The size of the baby's head being too small does not relate to the shoulder becoming stuck, and an umbilical cord wrapped around the neck does not necessarily result in shoulder dystocia. Excessive amniotic fluid can lead to other issues, but it is not a primary cause of the shoulders becoming trapped. Understanding these nuances is critical in recognizing the specific challenges associated with shoulder dystocia.